Most integrated circuits are today made of silicon, a material which is well understood and offers many fabrication advantages. It is recognized, however, that certain semiconductor compounds composed of elements from groups III and V of the periodic table, known as III-V materials, offer significant advantages over silicon. III-V materials such as gallium arsenide have a higher electron mobility than does silicon and devices made from it are therefore capable of operating at a higher speed. Band-gap characteristics of III-V materials typically make them more suitable for optoelectronic or photonic applications; for example, they can be efficient light emitters and light detectors at wavelengths for which silicon would be unsuitable.
One problem in the fabrication of III-V devices is the difficulty of growing large single-crystal ingots with a sufficiently low defect density; thus, it is impractical to make wafer substrates of III-V material with as large a diameter as that of silicon wafers. It has therefore been suggested that large-area III-V wafers, from which III-V devices are to be made, can be made by epitaxially growing III-V material on a planar surface of a relatively large-diameter single-crystal silicon wafer. Epitaxial growth refers to a method of depositing a material on a substrate such that the crystal structure of the deposited material effectively constitutes an extension of the crystal structure of the substrate. Vapor of the material is normally exposed to the substrate in a vacuum chamber that has been evacuated to an ultra-high vacuum condition, which may be defined as a condition in which the pressure is less than about 10.sup.-9 torr and typically about 10.sup.-10 torr. Under this and other appropriate conditions, successive monolayers of the material are deposited essentially as a single crystal on the substrate. Other advantages of growing III-V material on silicon result from improved ruggedness and thermal conduction properties of the resulting III-V devices. Moreover, such construction would make possible the integration of III-V devices with silicon devices.
One way matching the lattice structure of the deposited material to the substrate, as required for epitaxial growth, is by including an intermediate or transition epitaxial layer of a material such as germanium or calcium fluoride between the silicon substrate and the III-V epitaxial layer. The transition layer also alleviates the problem of thermal mismatch of the III-V material with respect to the substrate. It is normally important that the epitaxial growth of the transition layer and a III-V layer be done sequentially, and that steps are taken to avoid contamination of one material by the other. Thus, after an epitaxial layer of, say, germanium is grown over the silicon substrate, it is important that the ultra-high vacuum chamber be thoroughly exhausted of germanium gases before the step of epitaxially growing the III-V material, such as gallium arsenide. One problem with this method is that, without a high degree of care, which may be time-consuming, spurious germanium gases or particles may remain in the vacuum chamber during the gallium arsenide growth process which are likely to contaminate the gallium arsenide epitaxial layer. Another problem is that removal of the wafer into the outside atmosphere subjects it to a further risk of contamination and is also somewhat time-consuming. Any unwanted contamination of the III-V material that constitutes an active portion of a finished III-V device can severely degrade the operation of the device.